Figure 5. A root system in layered soil viewed as a resistance network

Figure 4. Factors influencing the generation of chemical information

Microscopic models

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6

Sink: Atmosphere V(t)

Transpiration

Water Stress

LAI

Absorbing

Plant Water

Roots r2

Rain/Irrigation

Root Length

Root Uptake

Resistance

Potential Gradient

Soil Water

Bottom of Root Zone

Drainage Rate

Matric Potential

H2O

Drainage

Figure 5. A root system in layered soil viewed as a

Figure 6. Water flow in the soilroot continuum. The

resistance network. Water held in capacitors moves

"valve" symbols represent water flow rates between

rectangular storage compartments or to external "sink"

through variable resistance. Diodes permit flow into the

roots but not from roots into the soil (after Hillel 1977).

in the atmosphere or groundwater (after Huck and Hillel

1983).

Table 3. Plant resistance, Rp, data of various crops extracted from litera-

ture (after Feddes 1981).

Type of

Root depth

(103 cm)

Crop

experiment

(cm)

Source

Birds foot trefoil

Lab

11.2

Gardner and Ehlig (1962)

Grass

Field

10.4

Rijtema (1965)

Potatoes

Field

10.3

Endrodi and Rijtema (1969)

Red cabbage

Field

4282

31.436.3

Feddes (1971)

Wheat

Lab

921

Yang and de Jong (1971)

Wheat

Lab

3158

Yang and de Jong (1972)

Italian ryegrass

Lab

517.5

Hansen (1974)

Maize

Greenhouse

Reicosky and Ritchie (1976)

Grain sorghum

Field

100

Reicosky and Ritchie (1976)

Douglas fir

Field

Nnyamah (1977)

lished literature that measurement of resistances

change to vapor and diffuses through the stomata

encountered in the soilrootplant system is still

and the leaf/air boundary layer. A generalized

difficult or even impossible for many crops.

overview of macroscopic water flow processes is

The pathway of water movement is from bulk

illustrated in Figure 6. Numerous water uptake

soil to the rhizosphere, across the rhizosphere

models are published in the literature and can be

and the soil/root interface, across the cortex and

classified as "macroscopic" or "microscopic."

the endodermis, and then into the lumen of the

xylem. Water then moves along the xylem to the

Macroscopic models

substomatal cavities where it undergoes a phase

Macroscopic models assume the vertical tran-

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